I also use Draw.io and recommend it highly. It's web-based, so it's also cross platform.

However, Lucidcharts has a better UI in that I can build diagrams after in LC than Draw.io. https://www.lucidchart.com/pages/

LC is paid if you need more than 1 chart (who doesn't?). But Draw.io is free. LC is also web-based and is cross-platform.

I've heard OmniGraffle is also excellent (never used it myself) but it only works on OSX/iOS: https://www.omnigroup.com/omnigraffle/